A useful method of electrostatographic imaging involves the deposition of a uniform layer of toner particles on the surface of an electroconductive element, followed by imagewise heating of the toner particles to tack them to the surface of the element, removal of the toner particles from the non-image areas, and subsequent fusing of the imagewise distribution of toner particles to form the desired image. The toner particles carry an electrostatic charge, whereby they are attracted to the electroconductive element by electrostatic forces. To be useful in such a process, the electroconductive element must exhibit an adequate degree of electrical conductivity combined with surface characteristics which facilitate uniform deposition of the layer of toner particles and effective adherence of the toner particles in the desired imagewise distribution.
The electroconductive element must also exhibit excellent transparency to both visible and UV light since a preferred application for the imaging element is as a reproduction image master for exposing, for example, lithographic printing plates. Since the apparatus used to expose lithographic printing plates typically utilizes lamps which emit visible and, especially, UV light an image master which has poor visible and UV light transparency will result in excessively long exposure times.
Electroconductive elements for use in the aforesaid electrostatographic imaging process can be comprised of an insulating support, such as a polyethylene terephthalate film, an electrically-conductive layer overlying the support, and a layer of a thermoplastic polymer overlying the electrically-conductive layer. Toner particles can be uniformly deposited over the surface of the layer of thermoplastic polymer by suitable applicator means, such as an electrically-biased magnetic brush applicator, and imagewise exposed by suitable heating means, such as a laser, that will tack the particles to the surface and then the element can be cleaned of unexposed toner particles by suitable means such as the vacuum sweeper device described in U.S. Pat. No. 3,410,203, issued Nov. 12, 1968, or the magnetic brush cleaning apparatus described in U.S. Pat. No. 5,138,388, issued Aug. 11, 1992. The electrically conductive layer permits the use of a suitable bias voltage during application of the uniform layer of toner particles and during removal of the excess toner particles.
It is particularly difficult in preparing an electroconductive element of the type described hereinabove to simultaneously meet the strict requirements for conductivity and transparency in the electrically-conductive layer.
A wide variety of electrically-conductive materials have been proposed for use in forming electrically-conductive layers in imaging elements. While many of them are capable of meeting one or more of the requirements of this invention, it is extremely difficult to simultaneously meet all of the requirements.
Conductive layers comprising ionically conductive materials such as inorganic salts, colloidal silicas, polymeric salts such as styrene sulfonic acid salt homopolymers and interpolymers are well known in the art. These materials may provide very transparent coatings. However, the conductivity of such materials is very humidity sensitive and at low humidity they are not sufficently conductive for the application of the present invention.
Conductive layers comprising semiconductive metal salts such as cuprous iodide described in U.S. Pat. Nos. 3,245,833, 3,428,451, and 5,075,171, for example, reportedly provide resistivities less than 1.times.10.sup.7 .OMEGA./sq. However, these conductive layers have significant absorption to UV light and are therefore not desirable for use in the present invention. In addition, these cuprous iodide layers are typically applied from harmful solvents such as acetonitrile, which also makes them undesirable.
Conductive layers comprising inherently conductive polymers such as polyacetylenes, polyanilines, polythiophenes, and polypyrroles are described in U.S. Pat. No. 4,237,194, JP A2282245, and JP A2282248, but, these layers are highly colored, and thus unsuitable for use in this invention.
Conductive fine particles of crystalline metal oxides dispersed with a polymeric binder have been used to prepare humidity insensitive, conductive layers for various imaging applications. Many different metal oxides are alleged to be useful as antistatic agents in photographic elements or as conductive agents in electrographic elements in such patents as U.S. Pat. Nos. 4,275,103, 4,394,441, 4,416,963, 4,418,141, 4,431,764, 4,495,276, 4,571,361, and 4,999,276. Preferred metal oxides are antimony-doped tin oxide, aluminum-doped zinc oxide, and niobium-doped titanium oxide. However, these materials do not provide acceptable performance characteristics in the demanding application of the present invention. In order to obtain high electrical conductivity, a large amount (100-10000 mg/m.sup.2) of metal oxide must be included in the conductive layer. This results in decreased transparency for thick conductive coatings. The high volume fraction of the conductive fine particles in the conductive coating needed to achieve high conductivity also results in brittle films subject to cracking and poor adherence to the support material.
Fibrous conductive powders comprising, for example, antimony-doped tin oxide coated onto non-conductive potassium titanate whiskers have been used to prepare conductive layers for photographic and electrographic applications. Such materials have been disclosed in U.S. Pat. Nos. 4,845,369, 5,116,66, JP A63009656, and JP A63060452. Layers containing these conductive whiskers dispersed in a binder reportedly provide improved conductivity at lower volume fractions than the aforementioned conductive fine particles as a result of their higher aspect (length to diameter) ratio. However, the benefits obtained as a result of the reduced volume fraction requirements are offset by the fact that these materials are large in size (10 to 20 .mu.m long and 0.2-0.5 .mu.m in diameter). The large size results in increased light scattering and hazy coatings. Reducing the size of these particles by various milling methods well known in the art in order to minimize light scattering is not feasible since the milling process erodes the conductive coating and therefore degrades the conductivity of these powders.
Transparent, binderless, electrically semiconductive metal oxide thin films formed by oxidation of thin metal films which have been vapor deposited onto film base are described in U.S. Pat. No. 4,078,935. The resistivity of such conductive thin films has been reported to be 10.sup.5 .OMEGA./sq. However, these metal oxide thin films are unsuitable for laser printer media applications since the overall process used to prepare them is complex and expensive and adhesion of these thin films to the film base and overlying layers is poor.
Equally as important as the electrically-conductive layer is the thermoplastic dielectric imaging layer which must function in conjunction with the electrically-conductive layer to provide the desired combination of properties. Thus, the present invention is critically dependent on the combined characteristics of the electrically-conductive layer and the imaging layer.
It is toward the objective of providing an improved electroconductive imaging element for use in electrostatographic imaging that the present invention is directed; in particular, an improved electrostatographic imaging element that is substantially free of mottle, has a very low UV-density and very low visible-density and has surface properties which facilitate the uniform deposition of a layer of toner particles and the subsequent formation of an image by imagewise heating of the toner particles.